This invention relates to a valve operating system for a multi-valve engine and more particularly to an improved valve actuating arrangement for such engines.
The use of multiple intake valves for each cylinder of an engine is well acknowledged as a way to increase the performance of the engine. It has been found that a larger number of smaller diameter valves can provide a greater flow area and less inertia than the use of single large valves. Thus, it is common with many engines, particularly high-performance engines, to use four valves per cylinder.
Generally, there are provided two intake valves on the intake side of the engine and two exhaust valves on the exhaust side for each cylinder. With this type of an arrangement, fairly large flow areas can be provided and engine power output increased for a given displacement for the reasons which have been noted above. The use of multiple valves for each cylinder of the engine, however, presents some problems.
As the number of valves is increased, there becomes an increasing problem in conjunction with the placement of the valves, the maintenance of low-combustion chamber volumes and low surface areas and the necessity to reduce the likelihood of interference between the valves and the piston. Thus, the valve size, number and placement has been generally a compromise between these factors.
It has also been acknowledged that performance can be further increased through the use of five valves per cylinder. With such an arrangement, three intake valves and two exhaust valves are provided for each cylinder. The general positioning of the intake valves is that there are two side intake valves that lie close to or actually extend over a plane containing the cylinder bore axis of the engine and which divides the intake side from the exhaust side. A third center intake valve is disposed between the side intake valves and further from the aforenoted dividing plane. The problems with maintaining small combustion chamber volumes to maintain high compression ratios, and low combustion chamber surface areas, to reduce the likelihood of quenching, are further aggravated when five valves per cylinder are employed.
As has been noted, it is frequently the practice to provide an arrangement wherein the intake valves in a five-valve arrangement extend over onto the exhaust side of the engine. As a result of this, the exhaust valves will have a smaller area than the side intake valves and then the total intake valve area. The use of smaller exhaust valves than intake valves is generally accepted. This is partially because there is a larger pressure difference on the exhaust side than the intake side and thus the exhaust charge may be discharged easier than the intake charge may be drawn into the combustion chamber. However, when the number and size of the intake valves is further increased through the use of three intake valves versus two exhaust valves, then an uneven flow through the combustion chamber may result.
It is, therefore, a principle object of this invention to provide an improved engine arrangement for a multi-valve engine and a valve actuating mechanism therefor wherein the flow through the combustion chamber is optimized.
When three intake valves are used for the engine, the side intake valves tend to promote a swirling motion that occurs about an axis that extends transversely to the axis of the cylinder bore. This action is normally called tumble. However, the center intake valve will create a motion which is in the opposite direction to the tumble generated by the side intake valves. This can tend to cause conflicting flow patterns and may at times reduce the turbulence in the combustion chamber under conditions when turbulence may be desirable. Therefore, it has been proposed to move the center intake valve closer to the side intake valves and to have its diameter being much smaller than the side intake valves. This gives rise to further flow problems in the combustion chamber and the necessity for controlling the flow in a way in addition to the size of the valves.
It is a further object of this invention to provide a valve actuating mechanism and valve arrangement for a multi-valve engine wherein maximum flow can occur with minimum obstruction.
It is a further object of this invention to provide a valve operating system for a multi-valve engine wherein the flow can be optimized.
Generally, the operating mechanism for the valves of an engine provides relatively symmetric opening and closing movement of the valves. That is, the mechanism which operates the valves, be it direct actuation or actuation through rocker arms, causes a valve lift that progressively increases up until the valve is fully opened and then closes the valve with the rates of opening and closing being substantially the same. This does not always provide maximum charging efficiency.
It is, therefore, a still further object of this invention to provide an improved arrangement for operating the valves of an engine so as to obtain maximum charging and discharging of the combustion chamber.
In addition to the factors noted above, there are times during the valve stroke when the piston is at top dead center and either the intake and/or exhaust valves may be open or partially open. When this occurs, it is necessary to either recess the valves or provide cavities in the head of the piston for clearance purposes. This is not only an expensive arrangement, but also increases the combustion chamber volume at top dead center and thus reduces the possible compression ratio. This decreases thermal efficiency.
It is, therefore, a still further object of this invention to provide an improved valve actuating mechanism for a multi-valve engine wherein clearances may be maintained and high compression ratios obtained without adversely effecting the flow pattern into the combustion chamber.
As an example, in order to improve high-speed charging performance, it is a normal practice to open the intake valve either at or before top dead center. When this is done, however, the pressure in the combustion chamber may be relatively high and exhaust gases can in fact backflow through the induction system. In addition, the clearance problems aforenoted may be existent. However, it has been the practice to embody such a valve timing in order to ensure good charging efficiency.
It is, therefore, a still further object of this invention to provide an improved valve actuating mechanism for the intake valves of an engine wherein the opening of the intake valves may be delayed until the time when charging efficiency can be improved without decreasing the total charge flow to the engine.
In a similar manner, it has also been the practice to maintain the exhaust valve in an open condition until after the piston has passed its top dead center position. This overlap between the opening of the intake valve and the closing of the exhaust valve can provide rough running under low-speed and low-load conditions. In addition, the valve placement must be such so that there is clearance between the heads of the valves when they are all open and also this requires added clearance in the combustion chamber between the head of the piston and the valves.
It is, therefore, a still further object of this invention to provide an improved valve actuating mechanism for an engine and particularly for the exhaust valves thereof.
One common way in which the valves of an overhead valve, overhead cam engine are operated is through the use of directly actuated thimble tappets. These thimble tappets are supported in bores within the cylinder and engage the stems of the valves or are connected thereto in some manner for opening and closing the valves upon rotation of the camshaft. Of course, the tappet must have sufficient surface area for good contact with the cam lobes and to reduce wear. However, this arrangement may at times result in the provision of tappet bodies that are not as large as desirable. However, for clearance purposes, the diameter of the tappets cannot be increased with conventional engines.
It is, therefore, a still further object of this invention to provide a valve actuating mechanism embodying thimble tappets wherein the thimble tappets may be made of a large diameter without adversely effecting clearances.
One way in which the effective diameter of the tappet may be increased is by offsetting it relative to the axis of rotation of the camshaft. However, if the tappet bodies are so offset, then the placement of the valves may also be adversely effected.
It is, therefore, a still further object of this invention to provide an improved cam and tappet arrangement for an engine wherein the tappets may be offset relative to the cams without changing the location of the valves.
Where the tappet bodies are offset from the camshaft, it may be desirable to also offset them relative to the valve stems. However, this may provide a bending load on the tappets which tend to cause them to cock in their supporting bores and causes high frictional loads and possible damage.
It is, therefore, a still further object of this invention to provide an improved cam tappet and valve arrangement wherein offsetting is possible without increasing loading and causing the likelihood of undesirable load conditions.